Recently, an integrated olefin epoxidation process was proposed wherein a secondary alcohol such as isopropanol is air-oxidized to form an oxidant mixture comprised of acetone, isopropanol, and hydrogen peroxide, the acetone is removed from the oxidant mixture, and the resulting acetone-lean oxidant mixture is used to epoxidize an olefin such as propylene in the presence of a titanium silicalite catalyst (U.S. Pat. No. 5,384,418; incorporated herein by reference in its entirety). The acetone which is removed is recycled back to isopropanol by reacting with hydrogen in the presence of a catalyst. In the past, typical hydrogenation processes have required high pressures, normally above 1200-1500 psig. Substantial savings, particularly with respect to the cost of building commercial scale plants, could be realized if efficient hydrogenation at lower pressures were possible. For example, large capacity compressors are not needed at hydrogenation pressures less than 500 psig. Additionally, low pressure hydrogen streams are commonly available at nominal cost from existing commercial sources (i.e., as by-products of other chemical processes).
Hydrogenation of the ketone removed from the oxidant mixture is complicated by the fact that substantial quantities of water will generally also be present. I have discovered that the use of supported ruthenium catalysts results in initial hydrogenation rates and selectivities to alcohol which are sufficiently high to be commercially useful in hydrogenating aqueous acetone streams of this type. However, after several days of continuous use, the activity of such catalysts decreases to an unacceptable level. As such catalysts are quite expensive, simply replacing the spent catalyst with fresh catalyst is not practical. Moreover, any regeneration technique which requires the catalyst to be removed from the hydrogenation reactor or treated with expensive reagents or requires a series of lengthy steps will also not be attractive from a commercial point of view.
Moreover, the regeneration of hydrogenation catalysts is a highly unpredictable and uncertain art. A standard treatise on hydrogenation [Rylander, Catalytic Hydrogenation in Organic Syntheses, Academic Press, p. 4 (1979)] observes that "[i]t is not easy to enumerate catalyst poisons" since "[t]hey vary from reaction to reaction." Further, according to this reference, while "[l]ost catalyst activity can sometimes be restored by regeneration," it is also true that "[o]ne can never be certain in advance which procedures will work." The prior art has previously suggested that ruthenium hydrogenation catalysts could be reactivated by either heating under reduced pressure and thereafter reducing the catalyst (U.S. Pat. No. 4,331,557) or by treating with carbon tetrachloride (U.S. Pat. No. 4,322,315).